Syscalls pass by int64. The identifier of the called function is in eax, parameters
are in ebx, ecx, edx, esi, edi.

Syscall list :

id=eax	Name			Parameters				Description
  0		thread_exit		none					Signal kernel that current thread has finished
  1		schedule		none					Switch to next thread (might be the current one)
  2		thread_sleep	ebx: time (int) msecs	Tell kernel to put current thread to sleep
  3		process_exit	ebx: return value (int)	Tell kernel to end current process, cleaning up everything
  4		printk			ebx: addr of a string	Print a message to screen

If a processes wishes to exit with an error code, it HAS to use process_exit. thread_exit will do nothing.